Inside your Sinatra app, you just have to require the bundler setup:
require "bundler/setup"
require "sinatra"
get "https://stackoverflow.com/" do
"Hello world!"
end
Alternatively, if you don’t want to add the additional require "bundler/setup" at the top of your app, you can instead invoke sinatra via bundle exec (e.g. bundle exec ruby myapp.rb)
This assumes that you have a Gemfile in the root of your application. It might look like this:
source "http://rubygems.org"
gem "sinatra"
This also assumes that you’ve already installed bundler (gem install bundler) and that you ran bundle install to install all the gem dependencies.